Method of producing a vaccum treated effervescing boron steel

ABSTRACT

A method of producing an effervescing, low carbon, boron steel in which a vacuum treatment is used to kill a steel melt containing sufficient oxygen to produce an effervescing steel by the addition of carbon.

United States Patent [1 1 Oliver et al.

[ Aug. 12, 1975 METHOD OF PRODUCING A VACCUM TREATED EFFERVESCING BORON STEEL [75] Inventors: Evan M. Oliver, Bethlehem; Joseph V. Marsilio, Allentown, both of Pa.

[52] US. Cl. 75/49; 75/48; 75/129 [51] Int. Cl C2lc 7/10; C220 39/00 [58] Field of Search 75/129, 48, 49

[56] References Cited UNITED STATES PATENTS 3.702243 ll/l972 Miltenberger 75/48 3,754,899 8/1973 Kanter 75/129 Primary ExaminerP. D. Rosenberg Attorney, Agent, or Firm-Joseph J OKeefe; Michael J. Delaney; William N. Rice 5 7 ABSTRACT A method of producing an effervescing, low carbon, boron steel in which a vacuum treatment is used to kill a steel melt containing sufficient oxygen to produce an effervescing steel by the addition of carbon.

9 Claims, No Drawings METHOD OF PRODUCING A VACCUM TREATED EFFERVESCING BORON STEEL BACKGROUND OF THE INVENTION This invention pertains to a method of producing an effervescing steel by a method which includes purifying molten iron by the use of sub-atmospheric pressure and the addition of boron in its reduced state.

Conventional melting practice for the production of very low carbon, low residual steels has used a basic open hearth furnace and aimed at the production of unusually high temperatures to obtain the high degree of oxidation necessary for the production of low carbon steel. It is also known to use oxygen to supplement the air used in open hearth combustion to provide this high degree of oxidation. Basic oxygen furnace (BOF) melting utilizing relatively high purity oxygen to provide heat by the oxidation of silicon, manganese and carbon contained in blast furnace iron may be substituted for open hearth melting.

Conventional low carbon, effervescing steels containin g boron are known to resist aging. It is also known that these steels may be rolled into sheet to provide steel having good formability and good deep drawing characteristics. The usual constituents of these low carbon steels are about as shown below. It should be understood that all percentages referred to in this specification are based on weight.

Carbon 0.10% maximum Manganese ODS/0.50% Phosphorus 0.00 l /0.03 5% Sulphur 0.003/0.040% Silicon 0005/0357:- Oxygen 0.001% minimum Nitrogen (LON/0.010%

It is also known that small amounts of residual alloys may be detrimental to the properties of these steels and typical maximum permissible amounts are as follows:

Tin 0.01% Chromium 0.04% Molybdc- 0.01%

num

Copper 0. I()% In addition, small amounts of deoxidizers such as aluminum, boron, vanadium, columbium and titanium have been added to these low carbon steels in varying combinations for a variety of additional purposes such as combining with carbon and nitrogen.

Steels of these compositions have been produced with a wide degree of deoxidation practice varying from rimming, semi-killed, killed and including combinations such as mechanical and chemical capping the steels in molds.

A more recent development in melting practice is to supplement the prior art furnace melting with a vacuum treatment so as to effect a further reduction in the carbon and oxygen in the steel by chemical reaction (1) so as to produce carbon monoxide.

An illustration of this combination is provided by U.S. Pat. No. 3,183,078 issued to T. Ohtake et al for Vacuum Process for Producing a Steel for Non-Aging Enameling lron Sheets. The killed steels disclosed in this patent contain aluminum and titanium.

A recent illustration of this combination is provided by U.S. Pat. No. 3,792,999 issued to K. Ruttiger for Method of Producing a Drawing and Deep Drawing Steel Resistant to Aging, Particularly for Single-Coat Enameling. The steels of this patent contain boron.

In addition, U.S. Pat. No. 2,771,651 issued to E. R. Morgan et al. for Method of Preparing Non-Aging Steels discloses a method of adding a strong deoxidizer such as aluminum to chemically cap a rimming steel containing boron.

The low carbon, boron steels of the prior art have not been commercially successful because of the manner in which the steels respond to rolling, deep drawing and enameling. Some of the steels have been satisfactory but some heats have been either hot short during hot rolling, result in excessive amounts of scrap during deep drawing and/or are unsatisfactory because of enameling problems. At least a portion of these problems is believed to be associated with the use of deoxidizers in combination with boron which results in the formation of a fully killed product which has a relatively poor yield from ingot to product because of the use of sink heads or extensive cropping of ingots.

It is the primary object of this invention to eliminate the aforementioned difficulties by providing an improved method of producing an effervescing, low carbon, boron steel.

Another object of this invention is to provide a commercial method for the production of an effervescing, low carbon, boron steel in which the carbon-oxygenboron contents are controlled to provide good hot and cold rolling without encountering difficulties due to hot shortness.

Still another object of this invention is to provide a method for the commercial production of an effervescing, low carbon, boron steel having improved formability as shown by sheet steel having good deepdrawing characteristics.

Still another object of this invention is to provide a commercial method for the production of an effervescing low carbon steel in which the oxidation is controlled so that the boron addition will combine with carbon and iron as small spheroidal iron-boro-carbides so as to improve the quality of porcelain enamel coatings by avoiding the presence of large carbides at or close to the surface of the steel sheet.

SUMMARY OF THE INVENTION This invention is a method of producing an effervescing, low carbon, boron steel by the addition of carbon to a melt killed by a vacuum treatment.

In the production of low carbon, low residual, effervescent, boron steel it has not been previously recognized that a vacuum treatment will provide a very effective means for controlling the carbon content at a very low level so that the killed steel product of the vacuum treatment can be utilized in the production of effervescing boron steel having a closely controlled carbon content at a very low level. In the method of this invention, a vacuum treatment is utilized to reduce the carbon to practically zero, i.e. not greater than about 0.005 percent, in a melt having an oxygen to carbon ratio of at least 1.4: I.

In the method of this invention the steel is produced from a mixture of blast furnace iron and steel scrap selected so as to provide a low residual alloy content. The mixture is melted under highly oxidizing conditions to provide from about 0.03 to 0.08 percent carbon. This preferred, but non-critical range of carbon of 0.03 to 0.08 percent will provide a sufficient degree of oxidation so that a vacuum treatment will'result in reducing the carbon in the melt to about 0.005 percent. This reduction in carbon is due to the effect of the vacuum in enhancing the reaction of carbon and oxygen to produce carbon monoxide gas in accordance with chemical reaction (l):

-The production of carbon monoxide will stop when the contained carbon is reduced to about 0.005 percent and the melt will contain about 0.025 percent oxygen and be essentially killed.

The broad range of acceptable manganese content is from about 0.05 to 0.50 percent. For some applications the upperlimit of manganese should be about 0.20 percent maximum to' prevent an undesirable loss of hot strength. When the manganese falls below about 0.12 percent the steel may become hot short and cause difficulty in rolling. Apreferred range of manganese would be from about 0.12 to 0.20 percent.

In a melt containing more than about 0.08 percent carbon it may be necessary to add sufficient oxygen to provide an oxygen: carbon ratio of about 1.4:l to insure that the completion of the vacuum treatment will result in about 0.005 percent carbon and 0.025 percent oxygen. As is well known, oxygen may be added in either the elemental form as a gas or in the combined form such as an iron oxide, e.g. mill scale. Addition of oxygen in excess of the amount needed to provide 0.005 percent contained carbon after the vacuum treatment is undesirable because the excess oxygen may be detrimental to deep drawing.

Boron is an essential addition which is preferably added to the killed steel melt after completion of the vacuum treatment. The broad range of the boron addition is in an amount which will result in the recovery of 0.006 to0.020 percent boron. This amount of boron is in excess of the amount necessary to chemically combine with normal nitrogen (0.003 to 0.005 percent) expected in these steels and which may cause strain aging. This excess boron is available to chemically combine with the carbon to form very small spheroidal ironboro-carbides which are well dispersed throughout the steel and reduce porcelain enamel defects which are commonly associated with the large iron carbide particles which occur when boron is not present.

The killed steel containing no more than about 0.005% carbon and 0.025 percent oxygen is converted into an effervescing steel by the addition of carbon in an amount to recover 0.005 to 0.040 percent carbon. The added carbon reacts with the contained oxygen in the melt to evolve carbon monoxide and provide an effervescing steel which may be solidified in the usual manner.

The effervescing steels of this invention are usually solidified in cast iron molds and bottle top molds are preferred to provide a good yield from ingot to product without any significant losses due to cropping of sinkheads and unsound metal. It is also desirable to mechanically cap the ingots in bottle top molds.

A first specific example of the method of this invention is provided by the following description of melting a 300 ton commercial heat of steel. A mixture of hot metal and low residual scrap was melted in a basic oxygen furnace. The melt containing 0.042 percent carbon and 0.07 percent oxygen was tapped into a ladle and sufficient ferromanganese was added to provide 0.15 percent manganese. The melt was killed by subjecting to a vacuum for 28 minutes, which was followed by additions of boron and carbon. The effervescent melt was poured into bottle top molds which were hot rolled and cold rolled into sheet steel of the following composition:

Carbon 0.0 l 3% Manganese 0. l 5% Phosphorus 0.002% Sulfur 0.0 l 3% Copper 0.005% Oxygen 0.020% Nitrogen 0.005% Boron 0.009%

The mechanical properties of the annealed cold rolled sheet steel were:

Yield strength 25.000 psi Tensile strength 39,000 psi Elongation 47% Rockwell B Hardness 34 Another specific example of the method of this invention is provided by the following description of processing a 10 ton melt of steel in an electric arc furnace. The melt at tap contained 0.06 percent carbon and 0.047 percent oxygen and was subjected to a vacuum treatment for 10% minutes at which time the evolution of gas was complete and the melt was killed. The killed melt contained 0.005 percent carbon and 0.015 percent oxygen, and carbon was added to produce an effervescing steel containing 0.031 percent carbon and 0.011 percent oxygen. Boron was added to the melt during pouring into open top cast iron molds and the solidified ingots were rolled into sheet steel of the following composition:

Carbon 0.03 l Manganese 0. l 7% Phosphorus 0.008% Sulfur 0.0 l 8% Copper 0.06% Oxygen 0.009% Nitrogen 0.008% Boron 0.012% Molybdc- Less than 0.02% num Tin 0.002%

Chromium 0.04% Aluminum Lessthan 0.005%

The mechanical properties of the annealed, cold rolled sheet steel were:

Yield strength 28,700 psi Tensile strength 44.600 psi Elongation 4 l7:

We claim: 1. Method of producing a low carbon, boron sheet steel comprising:

a. providing a steel melt having low residual alloy content containing by weight no more than 0.0l percent tin, 0.04 percent chromium, 0.01 percent molybdenum and 0. l percent copper, and 0.03 to 0.08 percent carbon,

b. treating the steel melt with a vacuum to essentially complete the chemical reaction C O CO T by reducing the carbon to about 0.005percent by weight to leave an excess of oxygen in an amount greater than will provide a 02C ratio of at least c. adding boron to the vacuum treated melt in an amount sufficient to retain 0.006 to 0.020 percent by weight boron,

d. adding sufficient carbon to react with the residual oxygen in the melt so as to provide an effervescing steel melt containing 0.005 to 0.04 percent by weight carbon,

e. solidifying the effervescing steel melt, and I f. hot and cold rolling the solidified steel to form sheet.

2. Method of producing a low carbon, boron steel sheet for deep drawing, said method comprising:

a. providing a steel melt having low residual alloy content containing by weight no more than 0.01 percent tin, 0.04 percent chromium, 0.01 percent molybdenum and 0.10 percent copper, and 0.03 to 0.08 percent carbon,

b. treating the steel melt with a vacuum to essentially complete the chemical reaction C O COT by reducing the carbon to about 0.005 percent by weight to leave an excess of oxygen in an amount greater than will provide a QC ratio of at least 0. adding boron to the vacuum treated melt in an amount sufficient to retain 0.006 to 0.020 percent by weight boron,

d. adding sufficient carbon to react with the residual oxygen in the melt so as to provide an effervescing steel melt containing 0.005% to 0.04% by weight carbon,

e. solidifying the effervescing steel melt in molds and rolling to form sheet, and

f. hot and cold rolling the solidified steel to form sheet.

3. Method of producing a low carbon, boron steel sheet for deep drawing and porcelain enameling, said method comprising:

a. providing a steel melt having low residual alloy content containing by weight no more than 0.01 percent tin, 0.04 percent chromium, 0.01 percent molybdenum and 0.10 percent copper, and 0.03 to 0.08 percent carbon,

b. treating the steel melt with a vacuum to essentially complete the chemical reaction C O CO by reducing the carbon to about 0.005 percent by weight to leave an excess of oxygen in an amount greater than will provide a O:C ratio of at least 1.4: l

. c. addingboron to the vacuum treated melt in an amount sufficient to retain 0.006 to 0.020 percent by weight boron,

d. adding sufficient carbon to react with the residual oxygen in the melt so as to provide an effervescing steel melt containing 0.005 to 0.04 percent by weight carbon,

e. solidifying the effervescing steel melt in molds and rolling to form sheet, and

f. hot and cold rolling the solidified steel to form sheet.

4. The method of claim 1 wherein step (a) includes adding oxygen in an amount to provide an 01C ratio of at least 1.4:].

5. The method of claim 1 wherein manganese is added in an amount to provide up to 0.50 percent by weight manganese in the steel melt being treated with a vacuum.

6. The method of claim 1 wherein the melt is solidified in a mold.

7. The method'of claim 2 wherein the melt is solidified in a bottle top mold.

8. The method of claim 7 wherein the melt being solidified in a bottle top mold is mechanically capped.

9. The method of claim 6 wherein the addition of boron of step (c) is made in the mold.

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1. METHOD OF PRODUCING A LOW CARBON, BORON SHEET STEEL COMPRISING: A. PROVIDING A STEEL MELT HAVING LOW RESIDUAL ALLOY CONTENT CONTAINING BY WEIGHT NO MORE THAN 0.01 PERCENT TIN, 0.04 PERCENT CHROMIUM, 0.01 PERCENT MOLYBDENUM AND 0.10 PERCENT COPPER, AND 0.03 TO 0.08 PERCENT CARBON, B. TREATING THE STEEL MELT WITH A VACUUM TO ESSENTIALLY COMPLETE THE CHEMICAL REACTION C+O CO BY REDUCING THE CARBON TO ABOUT 0.005 PERCENT BY WEIGHT TO LEAVE AN EXCESS OF OXYGEN IN AN AMOUNT GREATER THAN WILL PROVIDE A O:C RATIO OF AT LEAST 1.4:1, C. ADDING BORON TO THE VACUUM TREATED MELT IN AN AMOUNT SUFFICIENT TO RETAIN 0.006 TO 0.020 PERCENT BY WEIGHT BORON, D. ADDING SUFFICIENT CARBON TO REACT WITH THE RESIDUAL OXYGEN IN THE MELT SO AS TO PROVIDE AN EFFERVESCING STEEL MELT CONTAINING 0.005 TO 0.04 PERCENT BY WEIGHT CARBON, E. SOLIDIFYING THE EFFERVESCING STEEL MELT, AND F. HOT AND COLD ROLLING THE SOLIDIFIED STEEL TO FORM SHEET.
 2. Method of producing a low carbon, boron steel sheet for deep drawing, said method comprising: a. providing a steel melt having low residual alloy content containing by weight no more than 0.01 percent tin, 0.04 percent chromium, 0.01 percent molybdenum and 0.10 percent copper, and 0.03 to 0.08 percent carbon, b. treating the steel melt with a vacuum to essentially complete the chemical reaction C + O -> CO by reducing the carbon to about 0.005 percent by weight to leave an excess of oxygen in an amount greater than will provide a O:C ratio of at least 1.4:1, c. adding boron to the vacuum treated melt in an amount sufficient to retain 0.006 to 0.020 percent by weight boron, d. adding sufficient carbon to react with the residual oxygen in the melt so as to provide an effervescing steel melt containing 0.005% to 0.04% by weight carbon, e. solidifying the effervescing steel melt in molds and rolling to form sheet, and f. hot and cold rolling the solidified steel to form sheet.
 3. Method of producing a low carbon, boron steel sheet for deep drawing and porcelain enameling, said method comprising: a. providing a steel melt having low residual alloy content containing by weight no more than 0.01 percent tin, 0.04 percent chromium, 0.01 percent molybdenum and 0.10 percent copper, and 0.03 to 0.08 percent carbon, b. treating the steel melt with a vacuum to essentially complete the chemical reaction C + O -> CO by reducing the carbon to about 0.005 percent by weight to leave an excess of oxygen in an amount greater than will provide a O:C ratio of at least 1.4:1, c. adding boron to the vacuum treated melt in an amount sufficient to retain 0.006 to 0.020 percent by weight boron, d. adding sufficient carbon to react with the residual oxygen in the melt so as to provide an effervescing steel melt containing 0.005 to 0.04 percent by weight carbon, e. solidifying the effervescing steel melt in molds and rolling to form sheet, and f. hot and cold rolling the solidified steel to form sheet.
 4. The method of claim 1 wherein step (a) includes adding oxygen in an amount tO provide an O:C ratio of at least 1.4:1.
 5. The method of claim 1 wherein manganese is added in an amount to provide up to 0.50 percent by weight manganese in the steel melt being treated with a vacuum.
 6. The method of claim 1 wherein the melt is solidified in a mold.
 7. The method of claim 2 wherein the melt is solidified in a bottle top mold.
 8. The method of claim 7 wherein the melt being solidified in a bottle top mold is mechanically capped.
 9. The method of claim 6 wherein the addition of boron of step (c) is made in the mold. 